At least 35 people were killed, including 10 children in Gaza and three in Israel, when hostilities broke out in the Palestinian conflict.
In the most intense violence in years, jets bombed Gaza and hundreds of rockets hit Israel.
Israel said it sent 80 jets to bomb Gaza and sent infantry and armaments to reinforce the tanks already assembled at the border.
Video footage on Tuesday showed three thick clouds of black smoke rising from a 13-story apartment and office building in Gaza when it fell over after being demolished by Israeli air strikes.
A multi-story apartment building in Gaza collapsed and another was badly damaged after being repeatedly hit by Israeli air strikes early Wednesday.
The Israeli military said the building in the Rimal neighborhood of Gaza City houses “several” Hamas offices, including offices for military research, development and military intelligence.
The residents of the block and the surrounding area had been warned, according to the military, to evacuate the area before the air strike.
A second residential and office building in the same neighborhood was badly damaged in Israeli attacks just before 2 a.m. on Wednesday.
Gaza Health Ministry officials put the death toll at 32, but a Hamas-affiliated radio station later said three other people, including a woman and a child, were in an Israeli air strike on an apartment above one shortly before 2 a.m. on Wednesday Restaurant killed.
Israeli political leaders and the military said they killed “dozen” militants and hit buildings used by Hamas.
Defense Secretary Benny Gantz said Israel has carried out “hundreds” of strikes and “buildings will continue to collapse”.
The Gaza Ministry of Health said that of the reported deaths, 10 were children and one was a woman.
Meanwhile, Palestinian militant groups fired several missile barriers on Tel Aviv and Beersheba.
The Hamas armed wing said it fired 210 rockets at Beersheba and Tel Aviv in response to the bombing of buildings in Gaza City.
In Tel Aviv, air raid sirens could be heard in the city.
Israeli ambulance service Magen David Adom said a 50-year-old woman was killed when a rocket hit a building in Tel Aviv’s Rishon Lezion suburb and two women were killed in rocket attacks on Ashkelon.
It was one of the worst offensive strikes between Israel and Hamas since a war in Gaza in 2014 and sparked international concern that the situation could get out of hand.
Middle East Ambassador for Peace Tor Wennesland said on social media: “Stop the fire now.
“We are escalating to war in full. Leaders on all sides must take responsibility for the de-escalation.
“The cost of the war in Gaza is devastating and is being paid for by ordinary people. The United Nations is working with all sides to restore calm. Stop the violence now.”
The violence followed weeks of tension in Jerusalem during the Muslim fasting month of Ramadan, with clashes between Israeli police and Palestinian protesters in and around the Al-Aqsa Mosque on the site venerated as the Temple Mount by Jews and as a Noble Shrine by Muslims.
These escalated in the last few days before a – now postponed – trial on a case that could end with Palestinian families evicted from homes in East Jerusalem that were claimed by Jewish settlers.
Israeli Prime Minister Benjamin Netanyahu previously warned that the militants would pay a “very high” price for the missiles that reached the outskirts of Jerusalem on Monday during a holiday in Israel commemorating the conquest of East Jerusalem in a 1967 war.
The Arab League, of which some members have warmed relations with Israel in the last year, accused it of “indiscriminate and irresponsible” attacks in Gaza and said it was responsible for the “dangerous escalation” in Jerusalem.
Hamas group leader Ismail Haniyeh said Israel “lit the fire in Jerusalem and Al-Aqsa and spread the flames to Gaza, so it is responsible for the consequences”.
Ismail said Qatar, Egypt and the United Nations had contacted to demand calm, but Hamas’ message to Israel was, “If they want to escalate, the resistance is ready, if they want to stop, the resistance is ready.”
The US said Tuesday that Israel had a legitimate right to defend itself against missile attacks, but put pressure on Israel over the treatment of Palestinians and said Jerusalem must be a place of coexistence.
Foreign Ministry spokesman Ned Price urged calm and “restraint on both sides”, saying: “The loss of life, the loss of Israeli life, the loss of Palestinian life, we deeply regret it.”
He added, “We are calling for this message of de-escalation so that this loss of life comes to an end.”